Mammut’s Sphere range has been around for a while and has a proven reputation for featuring superlight, high-quality sleeping bags that are lean on features but offer robust performance. So, you don’t get a neck baffle, the zip is ¾ length and small, and the fit is slim and fitted to minimise on extra fabric and therefore weight. That said, it does have a couple of neat features, specifically the zip garage and a little peak on the hood, which can be useful if draughts or condensation are a problem. After the OMM Mountain Raid the Mammut Sphere UL Spring is the leanest sleeping bag in our test, and it comes with a decent compression sack (Rab and Marmot: take note!) and so it is good for those who want to keep weight well down. Temperature ratings are decent (it has a -1 deg C comfort limit) and you can’t argue with the quality. What lets the bag down is the touch-test of the inner layer. It is of the shiny black variety; and while not uncomfortable per se, it does tend to get a little clammy. Plus the quirky little games Mammut has added to the labels inside the bag – while quite sweet – are utterly mystifying.

The Mammut Sphere UL Spring is a solid buy for lightweight fans who still want warmth. But comfort could be better, and the Mountain Equipment Xero does better for less cash and only a little more weight.